A known type of digitizer employs a tablet having a planar upper surface for supporting work sheets, e.g., drawings, charts, maps or the like. The tablet also has a generally planar grid of conductors underneath the work surface.
The conductor grid is composed typically of a plurality of straight, parallel, usually equi-spaced conductors extending in a horizontal or x-direction, and a plurality of straight, parallel, usually equi-spaced conductors extending in a vertical or y-direction. The rectangular portion of the tablet upper surface immediately above the conductor grid is called the "active area."
The digitizer also employs a pen-shaped pointer, commonly called a stylus. The stylus typically has an elongated, cylindrical body terminating in a conical tip. Near the tip, the stylus contains an electrical coil disposed concentrically with the central axis of the stylus body.
In generating a signal indicative of the position of the stylus, the digitizer applies an electrical signal, e.g., to the electrical coil, to induce signals in the grid conductors due to electromagnetic inductance therebetween. Then, the grid conductors are scanned by detection circuitry to yield a series of voltage pulses of various amplitudes corresponding to the location of the conductors with respect to the stylus coil. (Alternatively, in other digitizer designs, the electrical signals are applied to the grid conductors while the signals electromagnetically induced in the electrical coil are sensed.)
The voltage pulses obtained from the conductors (or, in the alternative version, from the coil) define a sinusoidal-shaped signal envelope or waveform, which can be analyzed by a computer to obtain a calculation of the position of the stylus tip on the tablet upper surface.
During use of the digitizer, an operator uses the stylus to trace, points or lines on the tablet work sheet. The digitizer regularly generates and stores data representing the positions of the stylus as it moves.
Ideally, the stylus is held during use perpendicular by to the upper surface of the tablet. In that way, the loops of the electrical coil remain generally parallel to the upper surface and directly over the tip of the stylus, thereby permitting accurate determinations of the stylus location.
Unfortunately, most users hold and move the stylus in the manner of a standard writing pen or pencil; that is, the body of the stylus is held in the fingers at some angle that is offset from the vertical. With this orientation, called "pen tilt," the stylus tip is at one point on the tablet while, due to the tilt, the "apparent" position of the stylus tip, as would be detected by the digitizer absent correction for pen tilt, is at another point on the tablet. The point on the tablet at which the stylus tip is actually located is called the "contact point."
The apparent position of the stylus is the point on the tablet corresponding to the projection of the tilted stylus central axis to the grid plane. The distance between the contact point and the apparent stylus position can be called "projection error." Unless the digitizer compensates in the position reading to account for projection error along each axis of measurement, such error could reduce significantly the digitizer's accuracy.
The problem of pen tilt in digitizers is not new, and methods for correcting it are known in the art. For instance, U.S. Pat. No. 3,873,770 issued to Ioannou discloses a method of providing digital position measurements with stylus tilt error compensation. As described in that patent, the voltage waveform from the digitizer typically has a pair of spaced characteristic peaks whose magnitudes correspond to pen tilt. The patent's technique provides an error correction quantity determined from the peak voltages. Unfortunately, under extreme conditions of pen tilt (where the peak voltages can be as much as three fold their values occurring when the stylus is normal to the tablet), conventional detection-circuitry amplifiers can saturate and clip the waveform peaks, thereby compromising the results of that patent's approach.
Commonly-assigned U.S. Pat. No. 4,939,318 (Watson) discloses another method of detecting and correcting for pen tilt in a digitizer. The approach disclosed in the Watson patent determines pen tilt correction by comparing the magnitudes of an induced signal waveform at "points" (i.e., voltages corresponding to specific conductors, or, simply stated, conductor voltages) at fixed distances on either side of the apparent pen position, and inside the waveform peaks mentioned above with respect to the Ioannou patent.
The technique taught in the Watson patent is generally satisfactory in compensating for pen tilt, particularly in digitizers having tablets employing inter-conductor spacings (i.e., the distance between the conductors in the x-direction or y-direction) up to a maximum of about 0.3 inch (0.7 cm).
The Watson approach encounters problems, however, in digitizer tablets having larger inter-conductor spacings. It has been determined empirically that, with a coil diameter of a fraction of an inch as is typically found in a stylus, the waveform peaks lie about one inch (2.54 cm) apart. With that the case, use of the Watson technique with tablets having larger inter-conductor spacings than about 0.3 inch typically yield insufficient data between the peaks of the waveform for accurate, pen-tilt-compensated determinations of stylus position.